The Walker-Anderson model has been proven to model the penetration of long rods into semi-infinite targets with great results. The equations of motion of the projectile are obtained by analytically integrating the conservation of momentum along the cylindrically symmetric axis of penetration. In this study, the original Walker-Anderson model is presented in detail and extended to include back surface bulging and breakout. To obtain the velocity in finite targets, the potential for the velocity in a semi-infinite target is multiplicatively blended with the potential for the velocity in an infinitely thin target. Different choices of these potentials are discussed. The extended model is capable of calculating the penetration history, as well as predicting the residual length and velocity of the projectile when the target fails. The implementation is primarily focused towards eroding projectiles, but a transition from eroding to rigid penetration is included.The extended model is compared to experimental data and agreement is relatively good, although more validations are needed. This study was made in collaboration with the Swedish Defence Research Agency, FOI, and is expected to lay the foundation for further generalizations of long-rod penetration.